The present invention is related to composite membranes useful in fluid separations and methods for their preparation.
Semipermeable composite membranes prepared from various synthetic polymeric compositions are used in various commercial and industrial applications for the separation of various components found in liquids or gases. Reverse osmosis and nanofiltration membranes are typically relatively thin in order to provide a relatively high rate of transport of the liquid or gas through the membrane, i.e., high flux rate.
Composite membranes having an ultraviolet (UV) curable polymeric film coating as a discriminating layer laminated onto a porous support material have been made and shown to be somewhat useful in the separation of liquids and gases, for example, as in U.S. Pat. No. 4,976,897. In this patent, a composite membrane is prepared by first coating a porous support with a resin mixture having a high viscosity due to a high molecular weight, i.e., the viscosity is at least about 35,000 to 500,000 centipoise, in order that a significant amount of the pores of the support are not filled. The coated support is then cured with UV light to form a discriminating layer on the support. Because of the high viscosity of the resin mixture, the resulting discriminating layer generally has a thickness of 60,000 Angstroms or greater.
Membranes having discriminating layers of 60,000 Angstroms or greater are not optimum for many liquid separation applications. This is due to the fact that thicker membranes usually inhibit flux. Therefore, it would be desirable to have a process which produces a UV cured composite membrane having a thinner discriminating layer in order to maximize the flux.